Turbine engine designers are continuously looking for new materials with improved properties for reducing engine weight and obtaining higher engine operating temperatures. Titanium alloys, and in particular, titanium aluminide (TiAl) based alloys, possess a promising combination of low-temperature mechanical properties, such as room temperature ductility and toughness, as well as high intermediate temperature strength and creep resistance. For these reasons, TiAl-based alloys have the potential to replace nickel-based superalloys, which are currently used to make numerous turbine engine components.
Vacuum induction melting is one method often used to make turbine engine components, such as airfoils, and generally involves heating a metal in a crucible made from a non-conductive refractory alloy oxide until the charge of metal within the crucible is melted down to liquid form. When melting highly reactive metals such as titanium or titanium alloys, vacuum induction melting using cold wall or graphite crucibles is typically employed. This is because melting and casting from ceramic crucibles can introduce significant thermal stress on the crucible, which can result in the crucible cracking. Such cracking can reduce crucible life and cause inclusions in the component being cast.
Moreover, difficulties can arise when melting highly reactive alloys, such as TiAl, due to the reactivity of the elements in the alloy at the temperatures needed for melting to occur. As previously mentioned, while most vacuum induction melting systems use refractory alloy oxides for crucibles in the induction furnace, alloys such as TiAl are so highly reactive that they can attack the refractory alloys present in the crucible and contaminate the titanium alloy. For example, ceramic crucibles are typically avoided because the highly reactive TiAl alloys can break down the crucible and contaminate the titanium alloy with both oxygen and the refractory alloy from the oxide. Similarly, if graphite crucibles are employed, the titanium aluminide can dissolve large quantities of carbon from the crucible into the titanium alloy, thereby resulting in contamination. Such contamination results in the loss of mechanical properties of the titanium alloy.
Additionally, while cold crucible melting can offer metallurgical advantages for the processing of the highly reactive alloys described previously, it also has a number of technical and economic limitations including low superheat, yield losses due to skull formation and high power requirements. Furthermore, undesirable thermal stresses may build up in the crucible during the melting and casting process, which can damage the crucible, resulting in cracking. More specifically, different regions of the crucible can experience different thermal stresses during the melting and casting process.
For example, the outside of the crucible typically heats up faster than the inside of the crucible due to induction coupling, and also cools down faster than the inside of the crucible after pouring. This temperature difference can shift the point region of maximum stress from the inside of the crucible, into the crucible wall, and drive cracks therethrough. As another example, during melting, there is typically not much thermal stress about the top of the crucible as there is generally no molten material in this region. However, during pouring, the molten metal will contact the top of the crucible, thereby increasing the thermal stresses present in this region of the crucible. As previously described, such thermal stresses, and changes on thermal stresses, can result in crucible cracking, which can shorten crucible life and negatively impact crucible performance.
Accordingly, there remains a need for methods for making refractory crucibles capable of managing the thermal stress generated during the casting of highly reactive titanium alloys.